In the field of data communication a need exists to enable a large scale signal processor to communicate with a large number of data communication lines at a high data rate. For example, an airline reservation system handling data inquiries from all over the United States may be controlled by a central reservation control system. Such system normally includes a signal processor which must be able to communicate with a large number of data communication lines through data terminal and data communication equipment. Such data communication usually employs protocols and signals referred to as RS232 communications. Normally the signal processor operates at a very high speed so that it may service a large number of data communication lines even when these carry data at high rates. It is important in such system that communication between the data communication lines and the central processor unit be maintained and made essentially fault free.
Failures may occur in any area and equipment redundancies are employed to safeguard the central processor system against failures. In the part of the system to which the communication lines are coupled, a need exists to be able to switch lines from one equipment to another either to accommodate a failure or for such other purpose as appears necessary for the management or control of the central processor. One operational mode known as broadcasting may involve the momentary transmittal of the same message to a large number of lines. It is important that such mode be easily and quickly established with a minimum interference with other regular operations.
When a large number of data communication lines are to be connectable with each line formed of a large number of wires such as in excess of twenty as is common with RS 232 communication, the number of crosspoints and controllable connecting points of wires escalates to a cumbersome number. For example, with a 256.times.256 matrix, the number of crosspoints for a full size matrix would be in excess of 65,000 and the number of controllable connecting points of wires is in excess of one million for RS 232 communication lines. Such large number of connecting points are difficult to package in a small space as would be needed for high speed transmission of data.
Systems have been proposed and employed whereby a large number of data communication lines are coupled to a central processor through a switch. One such switch multiplexes all of the lines on each side of a crosspoint at a rate selected so that the through-put rate of the data on each line may meet a minimum required level. Such switch is limited in the number of lines it can handle lest the through-put data rate of the lines drops below the minimum required level. Such rate limitation becomes burdensome when one line requires a high through-put rate and needs to be sampled at a higher frequency. Such higher sampling rate can be accommodated, but at the expense of correspondingly reducing the sampling rate for other lines whose through-put data rate is then reduced. Such systems also suffer from lack of expandability.
For example, a commonly used data rate currently is 9600 baud per second. Higher data rates are frequently encountered such as 19.2K baud and higher. In order to pass data for each line at a high rate, the sampling speed for all lines must be sufficiently high.
When a large switch is considered, for example one that is capable of connecting any one of 256 incoming lines to any one of 256 outgoing lines, with each line capable of carrying 19.2K baud per second, the sampling rate for the multiplexing of all 256 lines requires an extraordinarily high sampling frequency. Such sampling frequency would be at least about 10 MHz to meet minimum Nyquist sampling rate requirements and practically a higher sampling rate would be necessary. When a larger number of lines is contemplated for a conventional switch, a correspondingly higher sampling frequency would be needed. However, in such case the higher sampling rate quickly leads to physical size limitations due to signal delays, noise, etc. As a practical matter, therefore, the switch size in conventional switches is limited because the sampling frequency cannot be indefinitely increased.